The present invention relates to a proximity detector, intended especially for detection of obstacles in proximity to a moving object such as a wire-guided trolley, robot, machine element or for the detection of intrusion into a premises.
The French Patent Application FR-A-2 606 162 describes such a detector in which a tuned transmission circuit with a capacitive antenna is excited by a radio frequency oscillator producing a high-frequency excitation signal. Reception means comprising a coil, forming a selective coupler with a coil of the transmission circuit, pick up an electrical signal at the frequency of the oscillator, whose amplitude is a function of the tuning of the transmission circuit. When a sufficiently conductive body is placed at a distance from the antenna sufficient to modify the capacitance of the latter, a demodulator connected to the reception means detects a variation of the peak-to-peak voltage of the signal delivered by the reception means and produces an output signal which is a function of the distance from the antenna to the foreign body.
Such a detector has the advantage of relying on very simple technology and of thereby being inexpensive. However, it requires, for its implementation, precise zero setting, that is to say that the transmission circuit must be perfectly tuned to the frequency of the oscillator, failing which the distance measurement supplied by the output signal is marred by errors. Moreover, this measurement is also affected during the lifetime of the detector by drifts of all types (thermal, voltage, capacitance variation of the antenna, etc.) to which the various components of the detector are inevitably subjected. In practice, it turns out to be tricky to compensate for these various drifts, without reverting to relatively complex techniques which add excessively to the cost of the detector.